This invention relates to measuring instruments and more particularly to an instrument for remote measuring of AC resistance over long lines.
The control and study of large irrigation systems involves monitoring changes in moisture content of soil in a field under test. This is generally accomplished by using soil moisture test cells which are located at different points in the field under test. The test cells behave as varying resistance in series with varying capacitance with a change in soil moisture, and variations in soil moisture content is monitored by periodically measuring the impedance, or AC resistance, of each test cell. The test cells are comprised of two stainless steel grid cylinders, one centered inside the other, forming plates to which an AC excitation signal is applied for measuring the impedance. The cost of manually connecting an instrument to individual moisture cells at each test location is prohibitive when surveying large areas. Moreover, measuring the resistance of the test cells on an individual basis requires that a technician walk through the field under test, and it is possible that other test arrangements may be disturbed, adversely affecting the reliability of the test results.
Measuring instruments have been proposed for the purpose of remote measuring of the AC resistance of soil moisture test cells. One such instrument employed a balanced bridge technique to compensate for line impedance. However, both resistance and capacitance balancing were required to achieve a null, and false nulls were possible when the approximate capacitance was not known. Another instrument, designed to compensate for the influence of excitation applitude, series resistance and excitation frequency, employed a technique for rectifying and averaging measured AC resistance. However, this instrument provided no compensation for line capacitance. Neither one of these instruments performed entirely satisfactory.
Therefore a need exists for a portable instrument capable of measuring AC resistance over long lines, typically hundreds of feet in length, and providing reliable readings of the resistance measured.